Distance to fault measurements in cable tv networks

ABSTRACT

A TDR technique for performing in-service distance-to-fault measurements in cable TV networks is disclosed. Using a cable network tester configured to generate chirped probe pulses and to perform pulse-matched filtering and averaging of received echoes, network faults may be detected without interfering with the downstream reception. The probe pulse transmission may be timed to take advantage of the error correction coding in the network.

TECHNICAL FIELD

The present disclosure generally relates to testing and monitoring ofcable TV networks, and more particularly relates to devices and methodsfor locating faults in a cable TV network.

BACKGROUND

Providers of digital television (DTV) services typically use two-wayhybrid fiber-coaxial (HFC) networks, which are shared bi-directionalnetworks with point-to-multipoint transmission in the downstreamdirection using digital signals or a mix of analog and digital signals,and multipoint-to-point transmission in the upstream direction. Signalsare distributed via a fiber optic connection from a head-end to a nodethat converts the optical signal to an electrical signal and thendistributes the signals to residences via a tree and branch coaxialcable distribution network termed ‘cable plant’. At the subscriber side,terminal equipment supports the delivery of cable services, which mayinclude video, data and voice services, to subscribers via cable modems.

Delivery of data services over cable networks, and in particular cabletelevision (CATV) networks, is typically compliant with a Data OverCable Service Interface Specifications (DOCSIS®) standard. The term‘DOCSIS’ generally refers to a group of specifications published byCableLabs that define industry standards for cable headend equipment,such as Cable Modem Termination System (CMTS), and cable modem (CM)equipment. The physical layer specification of DOCSIS provides for theuse of frequency multiplexing and several specific forms of quadratureamplitude modulation (QAM) for both upstream (CM to headend) anddownstream (headend to CM) communications. Upstream and downstreamsignals occupy separate frequency bands called upstream and downstreamfrequency bands. Downstream information channel signals co-propagate inthe downstream frequency band, and upstream signals co-propagate in theupstream frequency band. The frequency separation of the upstream andthe downstream signals allows bidirectional amplification of thesesignals, which propagate in a common cable in opposite directions. Inthe United States, most of the cable equipment installed at the time ofthe writing complies with the DOCSIS 3.0 version of the DOCSIS standard,which provides for the upstream spectral band from 5 MHz to 42 MHztypically, with the downstream channels using 64-QAM or 256-QAMmodulation format and 6 MHz spacing within the downstream spectral bandspanning from 50 MHz to 860 MHz. The upstream channel widths areconfigurable and may take a set of define values between 200 kHz and 6.4MHz, each corresponding to a specific symbol rate, with the upstreamdata modulated with either QPSK, 16-QAM, 32-QAM, 64-QAM or 128-QAM.

The upstream and downstream signals are prone to impairments that mayoriginate at a plurality of network locations in the network. As theresult of the “tree” structure of the cable plant, there may be numerousdevices, cable segments and connectors located between the fiber opticnode and the end user. This provides for a plurality of locations were adefect can occur, resulting in either no service or a reduced service tothe end user. In order to ensure adequate performance, the cable plantneeds to be monitored and tested and the source of impairmentsidentified and located.

Tracing the source of impairment typically requires that a techniciantravels to different network locations and compares measurements tolocate the impairment. Portable network testing devices currently usedin the industry may help to identify certain types of defects in thecable plant by performing specific spectral and noise measurements inthe upstream and/or downstream directions using specialized testingmethods at different network locations. A number of tests can also beperformed to evaluate quality of digital TV signal transmission onhigher logical levels of data transmission, for example by measuringsuch parameters as carrier level or amplitude, modulation error ratio(MER), bit error rate (BER), ingress under carrier (IUC), and otherparameters. The measurements may be performed on channel-by-channelbasis, each channel diagnostic data being summarized on a separatescreen or data page viewed by the technician on the tester's visualdisplay.

One type of defects that may be particularly hard to locate are defectsthat lead to changes in impedance along the signal path in the cableplant. These defects may be caused by cable corrosion, which may resultfrom the cable getting scratched and the outer shield rusting away dueto exposure to water, “rodent chews”, crushed, pinched or kinked cables,opens, shorts or partials in the cable. Impedance changes may also becaused by set screws inside housings, i.e. the screws that connect thecenter conductor of the cable to internal circuits of the amplifiers,splitters, taps, and fiber nodes, becoming loose if they areunder-torqued or causing oxidation through the galvanic process if theyare over torqued. Defects of these types may be located using timedomain reflectometry (TDR), which may include launching a short pulseinto the cable and detecting reflections from the location of theimpedance change, with the time delay between the transmission and thereflection indicating the distance to the fault. This may, however,require that the service to the customers be disconnected during themeasurements so that the strong TDR pulses do not interfere with thedownstream TV signals at the end user locations, and the weak reflectedTDR pulses are not obscured by the upstream DTV signals from the endusers. As the service to many customers may be impacted due to the treestructure of the cable plant, cable operators are understandablyreluctant to perform such measurements due to potential customercomplaints. Another approach could be to replace all possibly suspiciousconnections, cables and/or devices hoping that the defected part isamong them. Drawbacks of this approach includes increased costs and thatthe root cause of the problem remains un-identified.

Accordingly, it may be understood that there may be significant problemsand shortcomings associated with current solutions and technologies forlocating impedance-changing faults in a cable TV network.

SUMMARY

Accordingly, the present disclosure relates to a method and device forlocating a fault in a cable TV network using chirped TDR pulses tospread TDR pulse energy over a relatively long time interval, andfurther using pulse-matched filtering to process signals returned fromthe cable network and to magnify echoes of the TDR pulses that may becomprised therein.

According to one aspect of the present disclosure, the method includesusing a cable network tester comprising a signal transmitter and asignal receive, the method further comprising: a) using the signaltransmitter of the network tester to generate a pulsed probe signal andto launch said pulsed probe signal into the cable TV network, whereinthe pulse probe signal comprises a probe pulse that is characterized bya probe signal frequency that sweeps across a pre-defined probefrequency band over the duration of the probe pulse; b) using the signalreceiver of the network tester to receive a return signal from the cableTV network; c) filtering the return signal with a matched filter that ismatched to the probe pulse; and d) analyzing the return signal toidentify one or more peaks therein corresponding to one or more echoesof the probe pulse reflected at a fault location in the network.

Another aspect of the present disclosure relates to a cable networktester for locating a fault in a cable TV network, comprising: a signaltransmitter configured to generate a pulsed probe signal and to launchsaid pulsed probe signal into the cable TV network, wherein the pulseprobe signal comprises a probe pulse that is characterized by a probesignal frequency that sweeps across a pre-defined probe frequency bandover the duration of the probe pulse; a signal receiver configured toreceive a return signal from the cable TV network and comprising amatched filter that is matched to the probe pulse for filtering thereturn signal therewith and for amplifying in magnitude one or moreechoes of the probe pulse in the return signal; and, a processor foranalyzing the return signal to identify one or more peaks thereincorresponding to the one or more echoes of the probe pulse reflected ata fault location in the network.

According to a feature of one or more embodiments disclosed herein, theduration of each probe pulse and/or a repetition rate of the probe pulsegeneration may be selected so that symbol errors that may be caused bycollisions of probe pulses with downstream signals at the subscriber endare correctable by an error correction scheme used by the cable networkin the downstream transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein will be described in greater detail withreference to the accompanying drawings which represent preferredembodiments thereof, in which like elements are indicated with likereference numerals, and wherein:

FIG. 1 is a schematic block diagram of a portion of a cable network;

FIG. 2 is a schematic representation of a chirped probe signal that maybe used for locating faults in a cable network;

FIG. 3 is a schematic block diagram of a cable network tester forlocating faults in a cable network;

FIG. 4 is a flowchart of an embodiment of a method for locating faultsin a cable network using chirped probe pulses;

FIG. 5 is a graph representing a matched filter waveform;

FIG. 6A is a schematic diagram illustrating signal connections through aY-type connector at the output of the cable network tester of FIG. 3;

FIG. 6B is a schematic diagram illustrating an example three-leg summingresistor network implementing the Y-type connector of FIG. 6A;

FIG. 7 is a flowchart illustrating processing of a return signal in anembodiment of the method of FIG. 4 for locating faults in a cablenetwork;

FIG. 8(A) is a graph illustrating a sequence of chirped probe pulseslaunched into the cable network;

FIG. 8(B) is a graph illustrating the slicing of a return signalcorresponding to the sequence of chirped probe pulses of FIG. 8(A),including directly received copies of the chirped probe pulses;

FIG. 9 is a schematic block diagram of the downstream signaltransmission in a DTV network.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular circuits,circuit components, techniques, etc. in order to provide a thoroughunderstanding of the present disclosure. However, it will be apparent toone skilled in the art that aspects of the present disclosure may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well-known methods,devices, and circuits are omitted so as not to obscure the description.The appearances of the phrase “in one embodiment” in various places inthe specification are not necessarily all referring to the sameembodiment, nor are separate or alternative embodiments mutuallyexclusive of other embodiments. Note that as used herein, the terms“first”, “second” and so forth are not intended to imply sequentialordering, but rather are intended to distinguish one element fromanother unless explicitly stated. Furthermore, the followingabbreviations may be used:

HFC Hybrid Fiber-Coaxial

LAN Local Area Network

ASIC Application Specific Integrated Circuit

FPGA Field Programmable Gate Array

DSP Digital Signal Processor

QAM Quadrature Amplitude Modulation

CPE Customer Premises Equipment

CMTS Cable Modem Termination System

RF Radio Frequency

RAM Random Access Memory

ADC Analog to Digital Converter

DAC Digital to Analog Converter

DOCSIS Data Over Cable Service Interface Specification

Exemplary embodiments described hereinbelow relate to a method and arelated apparatus for in-service location of faults in a cable TVnetwork using time domain reflectometry (TDR) with chirped TDR pulses.Advantageously, the method may be implemented in a multi-functionportable cable TV tester that can also perform other cable testfunctions known in the art, such as, for example, channel scan foractive upstream and downstream channels, upstream channel demodulationand error analysis, etc.

With reference to FIG. 1, there is shown a schematic diagram of anexemplary cable TV network 1 wherein embodiments of the presentdisclosure may be practiced. The exemplary cable TV network 1, which isalso referred to herein simply as the cable network 1, may be a localportion of an HFC network that delivers Cable Television (CATV) signals,including digital TV signals and data and control signals, to end users.It will be appreciated however that methods and techniques describedherein can also be used in other types of wired communication networks,including but not limited to those using QAM formats, such as forexample DVB-T based digital TV systems, OFDM-based networks, DSLnetworks, and other types of networks, possibly with modifications whichwould be evident to those skilled in the art on the basis of the presentdisclosure.

As illustrated in FIG. 1, a fiber-optic node 10 of the cable network 1includes a downstream (DS) optoelectronic converter 10A that convertsdownstream (DS) optical signals generated by a remote CMTS (not shown)into downstream electrical RF signals 11, and an upstream (US)electro-optic converter 10B that converts upstream (US) electrical RFsignals 13 into US optical signals for upstream transmission to theremote CMTS. The fiber-optic node 10 is coupled via a coaxial cable 12to a bidirectional amplifier 15, which amplifies the downstream RFsignals 11 for distribution to first and second groups of homes 50A and50B. The downstream RF signals 11 generated by the downstreamoptoelectronic converter 10A of the fiber node 10 are distributed to aplurality of end-of-the-line subscribers, or end users, at customerpremises or homes 53, via one or more trunk coaxial cables 44 andsubscriber taps 51. At the customer premises 53, the DS signals aredemodulated using cable modems (not shown). One or more two-way trunk RFamplifiers 40 may further be provided in each trunk cable 44 to suitablyamplify the upstream and downstream CATV signals on their way to andfrom the subscriber premises 53. The first and second groups of homes50A, 50B may send upstream signals 31A and 31B, respectively, which maybe combined by the signal transmitter 110 into the upstream RF signal 13propagating towards the fiber node 106 for delivering to the remote CMTSat the headend (not shown).

A cable network tester 100 may be connected to the cable network 1 at adesired test point where the tester can inject a probe signal 101 intothe network in the downstream or upstream direction and receive backfrom the cable network return signals 103. The cable network tester 100,which is also referred to herein simply as tester 100, may be configuredfor testing a signal path in the cable network 1, including determiningthe location of a fault in the cable network 1.

The cable network 1 may serve a large number of homes 53, which may beconnected by taps 51 to a plurality of different cable trunks 44 at aplurality of different locations. The trunk cables 44 may be buried intothe ground or they may be elevated above the ground on utility poles, ora combination of both. Various faults and defects can occur anywhere inthe network 1, resulting in a lost or reduced service to the end users53. Many types of defects typically cause an abrupt change in theimpedance along a signal propagation path in the network; such defectsor faults include kinks, breaks or rust in the coaxial cables, rusty orotherwise imperfect cable connections and connections within variouscable equipment along the signal path, etc. These and other types ofcable plant faults tend to reflect back RF signals incident upon themand may be discovered using principles of time domain reflectometry(TDR). The TDR typically involves sending a short impulse of a durationτ along a signal transmission path and waiting for any echoes of thepulse that may return back as the result of the impulse reflection fromthe locations of the abrupt impedance change in the transmission linecaused by a fault. The distance to the fault (DTF) l=v·T/2 may then beestimated by measuring the time T elapsed between the sending of theoriginal pulse and the reception of its echo based on an expected valuev of the speed of propagation of the TDR pulse along the transmissionpath. The range resolution δR˜vτ/2 of this technique is defined by thepulse duration τ, so that shorter pulses generally yield betterresolution.

One potential issue that may be associated with using the short-pulseTDR technique in a cable TV network such as the cable network 1 is thatthe TDR pulses of a sufficiently high energy may be needed due torelatively high cable losses and to detect low-reflectivity defects, andsuch pulses may interfere with the cable TV signals at the subscriberpremises. Another possible issue associated with using the short-pulseTDR technique in the cable TV network 1 is that the echoes of the TDRpulse may be difficult to detect in the presence of the US signals 31A,31B generated by the end user equipment at 53. These issues may beresolved by taking the network 1 out of operation during the TDRmeasurement, which is undesirable.

In order to address one or more of these issues, tester 100 mayimplement an improved TDR technique as described hereinbelow withreference to FIGS. 2-9. The technique includes launching a pulsed probesignal 101 that is comprised of one or more frequency-chirped probepulses 111 into a cable network, and monitoring a return signal 103 fromthe network for echoes of these frequency-chirped probe pulsesoriginating a network fault. The one or more frequency-chirped probepulses 111 are electrical pulses that are formed of an oscillatoryelectrical signal which oscillation frequency f is “swept”, i.e.continuously increased or decreased, in time during each pulse across apre-defined frequency sweep band (f_(max), f_(min)) between a minimumfrequency f_(min) and a maximum frequency f_(max). The frequency sweepband (f_(max), f_(min)) is also referred to herein as the probefrequency band. By way of example, tester 100 may be connected to cablenetwork 1 in a manner allowing it to inject the frequency-chirped pulsedprobe signal 101 into the downstream direction, and to receive thereturn signal 103 propagating in the upstream direction. The returnsignal 103 may generally include echoes of the probe signal 101reflected from various locations in the network downstream from tester100, and upstream signals generated by the end users. One advantage ofusing the frequency-chirped probe pulses 111 for locating faults in anoperational cable network is that the pulse energy may be spread over acomparatively longer time period, i.e. the pulse duration τ, which mayenable either reducing the power of the probe signal in order to lessenthe interference with the downstream signals for the end users, orincreasing the signal to noise ratio (SNR) when detecting the reflectedechoes, or both. The frequency-chirped probe pulses 111 may also bereferred to herein as the frequency-swept (probe) pulses 111 or simplyas probe pulses 111. In one embodiment the frequency f, which may alsobe referred to herein as the probe signal frequency, may be changinglinearly in time from f_(min) to f_(max) during the duration of thepulse, in which case the probe pulses 111 may be referred to as linearlyfrequency modulated (LFM) pulses or as the LFM TDR pulses. In otherembodiments, the dependence of the probe signal frequency on time f(t)within each probe pulse 111 may deviate from linear. An example of thepulsed probe signal 101 that is composed of a sequence of thefrequency-swept probe pulses 111 of a pulse duration τ and pulse periodP is schematically illustrated in FIG. 2.

The return signal 103 may be processed using a pulse compressiontechnique based on the known waveform of the probe pulse 111, whichenables achieving a higher time-bandwidth product than for an equivalentCW pulsed signal. This results in an impulse signal that combines thehigher signal to noise ratio (SNR) of a long duration pulse with thehigher range resolution of the short pulse. The range, ordistance-to-fault (DTF), resolution δR of this TDR technique may beestimated from the width B=(f_(max)−f_(min)) of the probe frequencyband:

δR=v/(2B)  (1)

By way of example, using chirped probe pulses 111 with B=80 MHz mayyield the range (DTF) resolution of about 1.5 m, assuming v˜0.85·c,where c is the speed of light in vacuum.

Referring now to FIG. 3, there is schematically illustrated a anexemplary block diagram of the cable network tester 100 that isconfigured for implementing an embodiment of the chirped-pulse TDRtechnique for determining fault locations in the cable network 1. Theexemplary cable network tester 100, hereinafter also referred to astester 100, may include a signal transmitter 110 coupled to an outputcable port 118, and a signal receiver 120 coupled to an input cable port128. The signal transmitter 110 includes a probe pulse generator 112 forgenerating the probe signal 101, which may be followed by a DAC 114 thatmay in turn be followed by an optional output filter 116, which in oneembodiment may be a low-pass filter having a bandwidth B. The signalreceiver 120 includes an optional input filter 126, which may be forexample a low pass filter having the same or different bandwidth as theoutput filter 116, and which may be followed by an ADC 124, which outputis passed to a matched filter 122. One or more memory devices 130,hereinafter referred to as memory 130, may further be provided and maybe operatively coupled to the pulse generator 112 and the matched filter122. A return signal processor (RSP) 150 may be coupled to the memory130 and configured for processing the return signal 103 after it passedthe matched filter 122 as described hereinbelow, which may includeprocessing the return signal for presenting to a viewer in a formsuitable for extracting ranging information therefrom, identifyinglocations of possible faults in the cable plant, and estimating the DTF.An output device 170, such as a suitable display, that is coupled to thereturn signal processor 150 may further be provided for outputting, e.g.displaying, results of the processing.

The return signal processor 150, pulse generator 112, and matched filter122 may be embodied using a single dedicated or shared hardwareprocessor or using multiple hardware processors, and/or a combination ofsoftware and digital hardware. Examples of hardware processors that maybe used to implement blocks 112, 122, 150 include digital signalprocessor (DSP), Application Specific Integrated Circuit (ASIC), Fieldprogrammable Gate Array (FPGA), network processor, system on a chip suchas an FPGA with integrated ARM or micro processor, Complex ProgrammableLogic Device (CPLD), Erasable programmable logic device (EPLD), Simpleprogrammable logic device (SPLD), or macrocell array. In one exemplaryembodiment, a hardware processor implementing the return signalprocessor 150, such as a DSP, a suitable microcontroller, or a generalpurpose processor, runs a software or firmware program or programsincluding computer instructions for performing one or more operationsdescribed hereinbelow with reference to blocks 250, 330, 340, 350 inFIGS. 4 and 7; functionality of this software program will becomeapparent from the description hereinbelow and may include one or more ofthe following: dividing a saved sample or duration of the return signalin tine-domain slices synchronized to consecutive probe pulses 111,averaging the signal slices to obtain a cable response characteristics,and calibrating the cable response characteristics. In one embodiment,this software program is executable by a hardware processor implementingRSP 150 and is stored in a non-volatile memory (not shown) that iscoupled to the hardware processor. In one embodiment, the pulsegenerator 112 and the matched filter 122 may be embodied as hardwarelogic, for example using an FPGA. The memory 130 may be embodied, forexample, as RAM or as a combination of RAM and a non-volatile memorydevice.

One or more modes of operation of tester 100 will now be described withreference to FIG. 3, and further with reference to flowcharts shown inFIGS. 4 and 7, which illustrate embodiments of the method for locatingfaults in a cable network using tester 100 according to the presentdisclosure.

Turning first to the flowchart of FIG. 4 while continuing to refer toFIG. 3, one embodiment of the method may start with an optional step oroperation 210 wherein a technician connects tester 100 to the cablenetwork 1 at a suitable location for launching the probe signal 101 intoa section of the network to be tested. This operation may be omitted iftester 100 is already connected to the network, for example if tester100 is installed in the network on a permanent or semi-permanent basis.When present, this step may include connecting tester 100 to a cableconnection point in the network using a suitable Y-type three-portconnector 160, if tester 100 has separate output and input ports 118,128 as illustrated in FIG. 3, and these two ports are separatelyutilized for the transmission of the pulsed TDR signal 101 and forreceiving the return signal 103. The three-port connector 160 includes afirst port 161, a second port 162 and a third port 163, and isconfigured so that a signal received in the first port 161 istransmitted out of the third port 163, and a signal received in thethird port 163 is transmitted out of the second port 162. Step 210 mayinclude connecting the first and second ports 161, 162 of the Y-typeconnector 160 to the output and input ports 118, 128 of tester 100,respectively, and connecting the third port 163 of the Y-type connector160 to a cable connection point in the cable network 1. In oneembodiment, the pulsed probe signals received in the first port 161 mayalso be coupled into the second port 162, which may be utilized in someembodiments for probe pulse—echo synchronization as describedhereinbelow. The Y-type connector 160 may be embodied, for example, as athree-leg summing resistor network of the type schematically illustratedin FIG. 6B.

Once the tester is connected to the cable network 1, the method proceedsto step or operation 220 wherein the signal transmitter 110 of thetester 100 generates the pulsed probe signal 101 and launches it intothe cable TV network through the output port 118 of the tester 100. Thepulsed probe signal 101 includes at least one probe pulse 111 duringwhich the probe signal frequency f sweeps across the pre-defined probefrequency band (f_(min), f_(max)), as described hereinabove. Oncelaunched into the cable plant, the probe pulse 111 propagates throughthe coaxial cables and installed equipment of the cable network 1,experiencing attenuation and reflections from impedance discontinuitiesin its path, such as those that may occur due to faults or imperfectcable connections. By way of example, FIG. 1 indicates the location of acable fault 33 where the probe pulses 111 are partially reflected backtowards the tester 100. The reflected echo or echoes of the probe pulse111 will propagate in the reverse direction, reaching the tester 100 asthe return signal 103, with a time delay T relative to the time of thetransmission of the probe pulse 111 that is indicative of the distanceto the fault from the tester. At step 230 the return signal 103 isreceived by the signal receiver 120 of the tester 100, where it may beoptionally passed through the input filter 126. The received returnsignal 103 is passed to the matched filter 122 at step 240. The matchedfilter 122 is matched to the probe pulse 111 for compressing thereceived echoes of the probe pulses 111 in time, for example asdescribed hereinbelow. At step 250, the return signal passed through thematched filter 122 is analyzed by the signal processor 150 to identifyone or more peaks therein corresponding to the echoes of the probe pulseor pulses 111 reflected at the fault location 33 in the cable network.

The generation of the probe pulses 111 and the matched filtering of thereturn pulses may each be performed either digitally or using suitableanalog circuitry. In the embodiment illustrated in FIG. 3, both theinitial generation of the probe pulses 111 and the matched filtering ofthe echoes is performed in the digital domain using one or more digitalprocessors embodying the probe pulse generator 112 and the matchedfilter 122, such as for example an FPGA or an ASIC. In the shownembodiment, the pulse generator component 112 may be in the form of alogic circuit, for example defined in an FPGA, that loads a digitalwaveform of the pulse from a binary file saved in memory 130, and sendsit to the DAC 114 for converting it into the analogue probe signal 103.The return signal 103 received by the tester 100 is digitized by the ADC124 prior to the matched filtering operation, which in the shownembodiment is performed in the digital domain by a digital logicembodying the matched filter 122, which by way of example may be definedin the same FPGA or ASIC or in a different hardware processor.

In one embodiment, the digital waveform of the probe pulse 111 may begenerated, for example using a computer code, as a burst of anoscillatory LFM signal, such as for example a sampled sine wave of adesired length, which frequency is changing linearly in time, and thensaved in a memory region 131 of memory 130 that is readable by the pulsegeneration logic 112. In operation the pulse generation logic 122 mayload the saved burst waveform, scale it as required and output to theDAC 114 for converting into the chirped probe pulse 111. The length andthe rate of frequency change of the saved digital waveform of the pulsemay be selected for a given sampling rate of the DAC 114 so as toprovide the pre-determined duration τ of the probe pulse and the desiredminimum and maximum values f_(min), f_(max) of the probe pulsefrequency. By way of example, f_(min) may be equal to zero, and f_(max)may be in tens of MHz or greater, and the digital waveform may beconverted to a desired bit depth prior to saving, for example 12 bitfull scale value. The maximum frequency f_(max), and therefore thebandwidth B=(f_(max)−f_(min)) of the probe pulses 111, may be limited bythe sampling rate of DAC 114 and the output filter 116. By way ofexample, the sampling rate of DAC 114 may be 204.8 MHz, which limitsf_(max) and the pulse bandwidth B to 102.4 MHz. The bandwidth B of thepulsed signal 101 at the output of tester 100 may be further limited bythe output filter 116. In some of the exemplary embodiments describedherein f_(max) is taken to be equal to about 80 MHz, although oneskilled in the art will appreciate that this is not a requirement, andother values of the maximum frequency f_(max) of the chirped pulses andof their bandwidth B may also be realized, as supported by the used DACand the output filter bandwidth. Further by way of example, the durationτ of the probe pulse or burst may be in the range of 2 to 5 microseconds(μs), but may also be outside of this range. Some of possibleconsiderations that may guide the selection of the probe pulse durationτ are described hereinbelow. In one embodiment, the pulse generator 112may generate a digital probe signal wherein the digital probe pulse isperiodically or aperiodically repeated, with a buffer of zeros ofsufficient length placed between the pulses. The length of the bufferspacing between the probe pulses may be selected so as to allow time forthe echoes of the probe pulse 111 to return into the tester unimpeded bythe next probe pulse 111 being transmitted.

In one embodiment, the matched filter 122 is a digital filter thatperforms, substantially, a cross-correlation of the received signal 103with a waveform that substantially reproduces the waveform of the probepulse 111; this cross-correlation may be computed in the digital domainby convolving the matched filter waveform with the digitized returnsignal, producing peaks at the time points of maximum correlation of thereceived signal 103 with the matched filter. As the result, echoes ofthe probe pulse 111 are compressed in time and increased in amplituderelative to all other signals that may be present in the return signal103, including the upstream signals from the subscribers. Thus, the useof the chirped probe pulses 111 in combination with the matchedfiltering of the returned signals 103 makes it easier to detect echoesof the probe signal in the presence of upstream TV signals from thesubscribers by selectively amplifying the echoes of the probe pulses.The matched filter waveform may be created, for example, by terminatingto the ground the third Y-connector port 163, which in operation wouldbe connected to the cable under test, and capturing in memory 130 a copyof a chirped probe pulse 111 that is generated by the signal transmitter110 and received in the input port 128 from the output port 118 throughthe bridge 160, after it is digitized by the ADC 124 without passingthrough the matched filter 122. The captured pulse is substantially theprobe pulse 111 generated by the signal transmitter 110 that is shapedby the input filter 126 and possibly transmission responses of othercomponents of the tester frontend. Optionally a Hanning or othersuitable window may be applied to the captured pulse for sidelobeattenuation as known in the art, and the resulting filter waveform savedinto a binary file. An example of the matched filter waveform, with theoptional Hanning window applied to it, is illustrated in FIG. 5.

In order to determine the time delay T between the launching of theprobe pulse 111 into the cable plant and the return of an echo pulsecorresponding thereto, the return signal 103 received from the cableplant should be synchronized to transmission of the probe pulses 111. Inone embodiment, tester 100 may include an internal clock 140 thatprovides a receiver—transmitter time synchronization, and may furtherinclude logic for marking the time of the probe pulse transmission inthe digitized received signal that is provided to the return signalprocessor 150. In the absence of such internal synchronizationmechanism, the desired synchronization may be facilitated by sending acopy of each probe pulse 111 from the output port 118 of the signaltransmitter 110 directly into the input port 128 of the signal receiver120 to provide the reference markers from which the delay T may bemeasured, for example as described hereinbelow.

Referring to FIG. 6A, in one embodiment the time synchronization betweenthe pulse transmission and echo reception events may be accomplishedusing an embodiment of the three-port coupler 160 that splits the probesignal 101 a received form the output port 118 of the signal transmitter110 in two, sending a copy 111 b of each probe pulse 111 launched intothe cable plant directly into the input port 128 of the signal receiver120. In this embodiment, the signal 103 a received by the signalreceiver 120 is a sum of the return signal 103 received from the cableplant and the direct signal 101 b received directly from the signaltransmitter 110, with the coupler 160 providing a shortcut between theoutput and input ports 118, 128 of the tester 100 circumventing thecable plant. Thus, in this embodiment the signal 103 a received by thesignal receiver 120 of tester 100 includes both the probe pulse copy 111b, which is received directly from the output port of the signaltransmitter 110 and may be referred to herein as the direct pulse, and areturn pulse 113, which is an echo of the probe pulse 111 resulting fromthe reflection of the latter at a fault in the cable plant, and which istypically much weaker than the direct pulse 111 b. The return pulse 113is also referred to herein as the echo pulse, or as the probe echo, orsimply as the echo. The process of synchronization of the pulsetransmission and reception events in the tester 100 may then include i)identifying the position or timing of the direct pulse 111 b in thereceived signal 103 a, ii) identifying the position or timing of acorresponding echo pulse 113 in the received signal 103 a, and iii)determining the time delay T therebetween.

In one embodiment, tester 100 may generate a sequence of the chirpedprobe pulses 111 as illustrated in FIG. 8(A), and perform an averagingof their echoes 113 to further suppress noise and increase the signal tonoise ratio (SNR). Note that upstream signals 31 from the subscriberstypically have characteristics of a random signal and therefore may alsobe suppressed by the averaging. The wait time P between the transmissionof consecutive probe pulses 111 in the probe signal 101 should exceed amaximum expected value Tmax of the time delay T between the transmissionof the probe pulse 111 and the receptions of its echo 113 for a desiredmaximum distance Dmax in the cable plant that tester 100 may probe. Byway of example, for Dmax=1000 feet, the wait time P between consecutivepulses should exceed Tmax=2Dmax/v˜2.4 μs.

Referring now to FIG. 7, in one embodiment of the method tester 100 mayimplement the following steps or operations when processing the receivedsignal 103 or 103 a that is received at the input port 128 of the signalreceiver 120. First, the received signal is digitized in step 310 usingthe ADC 124, which converts it into the digitized received signal 141.At step 320, this digitized received signal 141 is passed through thematched filter 122, which outputs a filtered received signal 143 whereinany signals that correlate with the probe pulse waveform are compressedin time and amplified in magnitude. FIG. 8(B) illustrates this filtereddigital received signal for the embodiment wherein the received signal103 a includes the direct probe pulses 111 b and their echoes 113received from the cable plant. The matched filter operation convertsthese direct pulses 111 b and echo pulses 113 into narrow peaks 211 and213, with the bigger peaks 211 typically corresponding to the directpulses 111 b, and the smaller peaks 213 in-between them corresponding totheir echo pulses 113 received from the cable plant.

From the output of the matched filter 122, a duration or sample of thefiltered received signal 143 that includes a desired number N≧1 of thedirect pulses 211 may be saved in a memory region 135 of memory 130 thatis readable by the signal processor 150. In one embodiment, the signalprocessor 150 may be configured, for example programmed, to process thesaved sample of the filtered received signal 143 as follows. At step330, the saved sample of the filtered signal 143 is divided in timedomain into a plurality of N>1 time-domain signal slices 222 ₁ to 222_(N) that are synchronized to the sequence of probe pulses 111, i.e.each starting at a same position relative to a corresponding directpulse peak 211. In one embodiment each of these signal slices, which aregenerally referred to herein as slices 222, includes one big peak 211corresponding to a direct signal 101 b and a certain duration of thereceived signal 103 a that includes one or more smaller peaks 213corresponding to the one or more echoes of the probe signal 101. By wayof example, each signal slice 222 may start with one of the direct-pulsepeaks 211 and have the same length or duration, i.e. have the samenumber of data points. The length, or duration, of each slice 222 may beselected so as to include echoes reflected anywhere within a desiredlength Dmax of the cable plant being probed.

The RSP 150 may include logic embodying a suitable algorithm foridentifying the big peaks 211 in the saved sample of the receivedsignal; this identification may be facilitated by ensuring that thedirect pulses 111 b received at the input port 118 exceed in amplitudeor height any signal received from the cable plant. In one embodiment,the process of identifying the direct-pulse peaks 211 in the filteredsignal 143 may first determine a maximum value in the saved sample ofthe filtered signal, and then find all peaks therein that exceed acertain percentage of the maximum signal value, for example are at least90% thereof. The positions of the big peaks 211 in the saved signalsample 143 provide reference time points for obtaining the desiredranging information, i.e. determining the time delay T between thetransmission of the probe pulses 111 and receiving back their echoes113, from which the DTF may be computed.

In some embodiments tester 100 may implement an internal timesynchronization between the transmission and reception events withoutrequiring the sending of a copy of the probe pulses 111 a from theoutput to the input port of the device. For example, a specific digitaltiming signal may be added to the output of the matched filter 122 everytime a probe pulse is generated by the pulse generator 112, to provide apulse transmission marker. The starting points of each slice 222 maythen be identified in the saved received signal by the processor 150 inthe absence of the big peaks 211 therein, with each slice 222 startingat a same position relative to a marker identifying the instance oftransmission of one of the probe pulses 111.

At step 340, the processor 150 may perform an averaging operation on thetime-domain slices 222 of the return signal to obtain an average signalslice 223 wherein noise is suppressed and the echo peak or peaks 213 maybe easier to identify. The number of signal slices N used in theaveraging may depend on the application, measurements conditions,strength of the back reflections in the cable plant, etc. Generally, thegreater is the number of slices N, the better is the noise suppressionand the longer is the measurement time and the stronger requirements onthe device memory and processing speed. By way of example, the number ofsignal slices N in the 50-100 range should result in an additional 6-7bits of DTF resolution, or about 36-42 dB SNR improvement.

In one embodiment, the method may include an optional calibration step350 which produces a cable plant response characteristics, or an echoresponse, 225 wherein the direct-pulse peak 211 is removed so thepresence of the strong direct pulse 111 b wouldn't obscure the muchweaker reflected signals 103 received from the cable plant, and thesmaller echo peak or peaks 213 would be easier to identify. This stepmay include subtracting from the averaged signal slice 223 a calibrationslice that includes the direct-pulse peak but not the return signal fromthe cable. This calibration slice may be obtained using a process thatis similar to the creation of the matched filter waveform. It mayinclude terminating to the ground the third port 163 of the three-portbridge 160, transmitting a suitably large number of the probe pulses 111a, and performing the operations 310-340 outlined hereinabove withreference to FIG. 7. The resulting direct response slice is saved in amemory region 133 of memory 130 and used as the calibration slice duringnormal operation as described hereinabove.

Once the calibration slice is subtracted out from the average slice 223,the resulting echo signal 225 may be processed, for example scaled, forpresenting to the user. In one embodiment, the echo signal 225 may belogarithmically scaled, such as using a 20 log 10 (x) operation toobtain a magnitude ranging response in dB. This raging response may beplotted for the user as a function of time or distance along the cableplant, and/or passed for further analysis to a suitable peak detectionalgorithm that the RSP 150 may implement.

One advantage of using the chirped probe pulses for fault location in acable network is that, by suitably selecting parameters of the probepulse signal 101 such as the pulse width τ, the pulse spacing P and thepulse bandwidth B, the TDR measurements described hereinabove may beperformed while the network 1 is in operation without significantlyaffecting subscriber services. Specific approaches to selecting at leastsome of these parameters will be described hereinbelow by way of examplewith reference to a cable TV network, such as that illustrated in FIG.1, that implements data services according to Data-Over-Cable ServiceInterface Specification (DOCSIS), and in particular using upstream anddownstream signals specified in DOCSIS 3.0, Annex B. It will beappreciated however that the approaches described hereinbelow can alsobe used in cable or other wired networks using alternative communicationformats, possibly with modifications that would be evident to thoseskilled in the art based on the current disclosure.

In one embodiment, the cable network 1 may be a digital cable TV networkwherein video and other information is transmitted to the end users overa plurality of downstream channels of a pre-defined width B_(ch) each,with information-carrying signals in each channel modulated withsequences of symbols of a specific modulation format and duration T_(s).One or more of these downstream channels may overlap in spectrum withthe probe pulses 111, which may therefore interfere with the downstreamsignals in those channels at a subscriber modem when received at thesame time. In accordance with one aspect of the present disclosure, theprobe signal frequency is swept during each probe pulse over the probebandwidth B that is greater than the channel bandwidth B_(ch) of thedownstream signals 11, which reduces the interference time t_(i) by afactor B_(ch)/B:

t _(i) =τ·B _(ch) /B.  (2)

Here the interference time t_(i), which may also be referred to hereinas the collision time, is the time that the energy of a probe pulse 111spends within the downstream channel bandwidth B_(ch). By selecting B tobe significantly greater than B_(ch), for example by a factor of 10 ormore, the interference time t_(i) made quite small, thereby reducingnegative effects of the interference on the subscriber. By way ofexample, B_(ch)=6 MHz, B=80 MHz, and the time t_(i) any particular probepulse may interfere with a downstream signal in any one downstreamchannel is shorter than the pulse duration τ by a factor of 0.075.Further by way of example, a probe pulse 111 of duration τ=2.48 μs mayonly affect any given downstream channel for a durationt_(i)=τ·B_(ch)/B˜0.186 μs, which compares to 0.18655 μs symbol durationT_(s) specified for 256QAM transmission in DOCSIS 3.0.

In some embodiments, tester 100 may be configured to generate the probesignal 101 that takes advantage of error correction coding that istypically implemented in digital communications, including digital TV(DTV) signal transmission. The error correction coding used in thedownstream signal transmission in DTV networks is typically designed forcorrecting bursts of errors up to a specific length, i.e. a maximumcorrectable burst duration T_(bsrt), which may depend on a particularerror correction coding scheme chosen for a given channel. Accordingly,in one embodiment the probe pulse duration τ and/or the probe pulsebandwidth B is/are selected so that the probe signal frequency f isswept over the channel bandwidth B_(ch) faster than the maximumcorrectable burst duration T_(brst), i.e.

t _(i) <T _(brst),  (3)

which corresponds to choosing the probe pulse duration that satisfiesthe following condition:

τ<T _(brst) ·B/B _(ch).  (4)

Referring to FIG. 9, the error correction schemes used in DTV networkstypically include using a forward error correction (FEC) encoder 410followed by an interleaver 420 at the DTV signal transmitter, and ade-interleaver 450 followed by a FEC decoder 440 in the receiver of thesubscriber's modem. By way of example, the downstream signaltransmission in DOCSIS 3.0 networks is specified in the InternationalTelecommunication Union (ITU) Recommendation J.83B, “DigitalTransmission of Television Signals, Annex B”, which is incorporatedherein by reference. This document specifies the use of either 64-QAM or256-QAM modulation formats, and a Reed-Solomon encoder/decoder incombination with a variable-depth convolutional (I, J) interleaver inthe downstream transmission. Table 1 lists some of the possible valuesof the number of taps I and the interleaving increment J, along with thecorresponding values of the burst protection interval T_(brst) (thirdcolumn) and the interleaver latency L=((I−1)·I·J·T_(s)) (fourth column)for the 64-QAM and 128-QAM modulation formats defined in the J.83BRecommendation.

TABLE 1 I J Burst protection Latency (# of Taps) (Increment)64QAM/256QAM 64QAM/256QAM 8 16 5.9 μs/4.1 μs 0.22 ms/0.15 ms 16 8  12μs/8.2 μs 0.48 ms/0.33 ms 32 4 24 μs/16 μs 0.98 ms/0.68 ms 64 2 47 μs/33μs 2.0 ms/1.4 ms 128 1 95 μs/66 μs 4.0 ms/2.8 ms 128 2 190 μs/132 μs 8.0ms/5.6 ms 128 8 759 μs/528 μs 32 ms/22 ms

By way of example, a DOCSIS 3.0 network may correct any potential errorsthat may be caused by a collision with a solitary probe pulse 111 up toabout 40-50 μs duration, for the probe pulse bandwidth B=80 MHz andassuming the worst-case burst correction case of 256 QAM and (8,16)interleaver.

The burst correction capabilities specified in Table 1 are generallyapplicable when the error bursts of the specified maximum size happeninfrequently. For example, they may not hold with respect to symbolerrors that may be caused by a periodic sequence of the probe pulseswith the period P that is smaller than the latency L of the interleaver420 used in the downstream channel transmission. By way of example, theinterleaver latency of the deepest (128,8) interleaver specified byDOCSIS 3.0 is about 32 ms for 64QAM DOSCIS signals, and about 22 ms for256QAM DOSCIS signals having a shorter symbol duration. In order toestimate potential effects of the more frequent probe signaltransmission on the subscribers of the cable services, the duration ofpotential interference t_(i)=τ·B_(ch)/B, i.e. the time interval duringwhich any given probe pulse 111 may overlap in frequency with adownstream channel, may be compared to the duration T_(s) of one symbolof the downstream DOCSIS signals. The probability of the probe pulsesleading to symbol errors at reception may be reduced, for example, byselecting the probe signal parameters τ and/or B so that t_(i) issignificantly smaller than T_(s).

Furthermore, we found that the risk of unrecoverable symbol errors dueto a longer probe pulse may be completely avoided, or at leastsignificantly reduced, by a proper timing of the probe pulsestransmission, for example by synchronizing them with the FEC codewordfrequency in the downstream channel. Indeed, it can be seen that byselecting t_(i)≦n·T_(s), where n=1, 2, . . . is an integer, no more than(n+1) consecutive downstream symbols may potentially experience aninterference with any given probe pulse 111 at the reception. Thisinterference may not lead to symbol errors after the de-modulation bythe user modem if the downstream signal received by the modem is strongenough so that it can be decoded in the presence of the interferencefrom the probe pulse. Furthermore, potential symbol errors caused bythis interference may be corrected by the FEC decoder at thesubscriber's modem if the number of symbols in each FEC codeword thatmay be affected by the probe signal 101 does not exceed the number ofsymbols m that the FEC decoder is capable of correcting. By way ofexample, the ITU Recommendation J.83B specifies the use of Reed-Solomon(RS) block (128, 122) code for FEC encoding/decoding, with a 128 symbollong codeword, or coding block, that is capable of correcting up to m=3symbol errors per codeword. The same RS code is used for both 64-QAM and256-QAM signals; the actual duration T_(CW) of each codeword in thedownstream signal is however different for the 64QAM and 256QAM signalsdue to the differing duration of individual symbols T_(s) for these twomodulation formats.

Accordingly, in one embodiment the probe pulses 111 are generated nomore frequently than one probe pulse per one FEC codeword of thedownstream channel signal, and so that, during each of the probe pulses111, the probe signal frequency f is swept across the channel bandwidthB_(ch) over a time interval t_(i) that is no greater than (m−1) symbolintervals T_(s) of the downstream digital signal. This correspond toselecting the bandwidth B and/or the probe pulse duration τ so that

t _(i) =τ·B _(ch) /B≦(m−1)·Ts,  (5)

or

τ≦(m−1)·T _(s) ·B/B _(ch)  (6)

By way of example for m=3, B_(ch)=6 MHz, B=80 MHz, and T_(s)=0.187 μs(256QAM signals), condition (6) yields that the duration of each probesignal 103 should not exceed about 4.97 μs. Further by way of example,the probe pulses are generated with the probe pulse period P notexceeding the RS codeword length or duration T_(CW), which is about47.756 μs for the 256QAM DOCSIS signal.

An analysis of the operation of the DOCSIS error correction schemesrevealed that the convolutional de-interleaver specified by DOCSIS 3.0may group together into a single codeword downstream symbols thatexperience collisions with different probe pulses even when the periodof probe pulse generation P somewhat exceeds the codeword durationT_(cd), but is smaller than the interleaver latency. However, thisgrouping does not occur if P=T_(cd), i.e. if the generation of the probepulses is synchronized with the codeword frequency F_(cd)=1/T_(cd) inthe downstream channel. Accordingly, in one embodiment tester 100generates the sequence of the probe signals 103 that is synchronized tothe codeword frequency F_(cd) in the downstream digital signal.

A complication may arise when two different downstream channels that liewithin the frequency band swept by the probe signal 101 utilize twodifferent transmission formats that have differing symbol rates andtherefore differing symbol and codeword durations. Accordingly, in oneembodiment tester 100 generates the sequence of probe pulses that issynchronized to the codeword frequency for one of the two transmissionformats that has a lower noise tolerance. By way of example, DOCSIS 3.0specifies symbol rate of 5.056941 MHZ for 64QAM signals, and a slightlyhigher symbol rate of 5.360537 MHz for 256QAM signals. The codewordrates for these downstream signals are approximately 19.7 KHz and 20.9KHz, respectively. However, the noise tolerance of the 256QAM signalsare about 6 dB lower than that of the 64QAM signals. Accordingly, inthis exemplary embodiment the probe pulses may be generated at the256QAM codeword rate of about 20.9 KHz, or at any higher-order harmonicsthereof.

The aforedescribed method for locating faults in the cable network maybe implemented, for example, in a multi-purpose portable cable networktester that may be used by a technician in the field, and that may alsoimplement other network test functions. For example, in embodimentswherein the probe pulse generator 112 and the matched filter 122 areimplemented using a programmable hardware logic device such as an FPGAand the RSP 150 is implemented using a hardware processor such as a DSPexecuting software instructions, the same FPGA and the hardwareprocessor may be shared to implement other network tests, such as forexample those disclosed in U.S. Patent Documents No. 2015/0009795,2015/0020129, and 2015/0009340, which are incorporated herein byreference. Implementing the aforedescribed TDR measurement technique ina portable cable network tester that can also perform measurements onupstream and downstream DTV signals may have an additional advantage ofenabling to adjust the TDR parameters, such as the pulse probe power,duration and periodicity, in dependence on the DTV signals that arepresent in the network, so as to facilitate non-interfering TDR. Forexample, in one embodiment step 220 of the TDR measurement illustratedin FIG. 3 may be preceded by using the tester 100 to measure the powerof the DS network signals 11 at the tester location in the DS channelsthat lie within, or overlap with, the frequency sweep (f_(min), f_(max))of the probe signal 101, and adjusting one of the probe pulse power, theprobe pulse duration τ, and the probe pulse period P in dependence onthe measurements. For example, the probe pulse duration τ may beincreased to gain SNR at the receiver 120. In one embodiment, tester 100may perform symbol timing measurements in the DS transmission to ensurethat the TDR probe pulse transmission is synchronized with theinterleaver codewords.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentdisclosure. Thus the present disclosure is capable of many variations indetailed implementation that can be derived from the descriptioncontained herein by a person skilled in the art. All such variations andmodifications are considered to be within the scope and spirit of thepresent disclosure as defined by the following claims. All statementsherein reciting principles, aspects, and embodiments of the disclosure,as well as specific examples thereof, are intended to encompass bothstructural and functional equivalents thereof. Additionally, it isintended that such equivalents include both currently known equivalentsas well as equivalents developed in the future, i.e., any elementsdeveloped that perform the same function, regardless of structure. Thus,for example, it will be appreciated that block diagrams herein canrepresent conceptual views of illustrative circuitry embodying theprinciples of the technology. Similarly, it will be appreciated that anyflow charts, state transition diagrams, pseudocode, and the likerepresent various processes which may be substantially represented incomputer readable medium and so executed by a computer or processor,whether or not such computer or processor is explicitly shown.

The functions of the various elements including functional blockslabeled or described as “processors” or “controllers” may be providedthrough the use of dedicated hardware as well as hardware capable ofexecuting software in association with appropriate software. Whenprovided by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared or distributed.Moreover, explicit use of the term “processor” or “controller” shouldnot be construed to refer exclusively to hardware capable of executingsoftware, and may include, without limitation, digital signal processor(DSP) hardware, read only memory (ROM) for storing software, randomaccess memory (RAM), and non-volatile storage.

Thus, the present disclosure is not to be limited in scope by thespecific embodiments described herein. Indeed, other various embodimentsof and modifications to the present disclosure, in addition to thosedescribed herein, will be apparent to those of ordinary skill in the artfrom the foregoing description and accompanying drawings, and such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Further, although the present disclosure hasbeen described herein in the context of a particular implementation in aparticular environment for a particular purpose, those of ordinary skillin the art will recognize that its usefulness is not limited thereto andthat the present disclosure may be beneficially implemented in anynumber of environments for any number of purposes.

We claim:
 1. A method for locating a fault in a cable TV network using anetwork tester, comprising: launching a pulsed probe signal into a cableTV network, wherein the pulse probe signal comprises a probe pulse thatis characterized by a probe signal frequency that sweeps across apre-defined probe frequency band over the duration of the probe pulse;receiving a return signal from the cable TV network; filtering thereturn signal with a matched filter that is matched to the probe pulse;and analyzing the return signal to identify one or more peaks thereincorresponding to one or more echoes of the probe pulse reflected at afault location in the network.
 2. The method of claim 1, whereinlaunching the pulsed probe signal comprises sweeping the probe signalfrequency over a probe signal bandwidth that is greater than a channelbandwidth of the downstream frequency channel transmitted by the cableTV network.
 3. The method of claim 2, wherein the cable TV networkutilizes error correction coding for the downstream digital signal thatis capable of correcting m symbol errors per a codeword, where m is aninteger greater than 1, and wherein launching the pulsed probe signalcomprises sweeping the probe signal frequency across the channelbandwidth over a time interval that is no greater than (m−1) symbolintervals of the downstream digital signal.
 4. The method of claim 3,wherein launching the pulsed probe signal comprises generating asequence of the probe pulses that is synchronized to a codewordfrequency in the downstream digital signal.
 5. The method of claim 3,wherein the cable TV network supports two different transmissionformats, and wherein launching the pulsed probe signal comprisesgenerating a sequence of the probe pulses that is synchronized to acodeword frequency for one of the two transmission formats that has alower noise tolerance.
 6. The method of claim 1, wherein the cable TVnetwork utilizes error correction coding for downstream digital signalsthat is capable of correcting burst errors up to a pre-determinedmaximum correctable burst duration, and wherein launching the pulsedprobe signal comprises sweeping the probe signal frequency over thechannel bandwidth faster than the maximum correctable burst duration. 7.The method of claim 6, wherein the error correction coding used in thedownstream transmission comprises using an interleaver, and whereinlaunching the pulsed probe signal comprises generating a sequence of theprobe pulses wherein two consecutive probe pulses are spaced at least bya time interval corresponding to a latency of the interleaver.
 8. Themethod of claim 1, wherein the cable network tester comprising a signaltransmitter for generating the pulsed probe signal and a signal receiverfor receiving the return signal, the method further comprising sending acopy of the pulsed probe signal generated by the signal transmitter tothe signal receiver as a timing reference.
 9. The method of claim 1wherein launching the pulsed probe signal comprises generating asequence of the probe pulses, and wherein analyzing the return signalcomprises: collecting a plurality of time-domain slices of the returnsignal synchronized to the sequence of probe pulses, each time-domainslice of the return signal including a duration of the return signalfollowing the transmission of one of the probe pulses, and averaging theplurality of time-domain slices of the return signal to obtain anaveraged slice of the return signal.
 10. The method of claim 9,comprising launching the sequence of the probe pulses into the cablenetwork over an output port of the cable network tester and receivingthe return signal over an input port of the cable network tester,wherein launching the pulsed probe signal further comprises sending acopy of the sequence of the probe pulses from the output port directlyinto the input port for combing with the return signal prior to thefiltering.
 11. The method of claim 10, wherein each of the time domainslices of the return signal comprises a copy of the probe pulsesreceived directly from the output port, and an echo thereof receivedfrom the cable network, and wherein analyzing the return signalcomprises aligning the time-domain slices in accordance with probe pulsepositions therein.
 12. The method of claim 10, further comprisingsubtracting a calibration copy of the probe pulse from the averagedslice of the return signal to obtain a time-domain impulse response ofthe cable network.
 13. The method of claim 10, comprising using athree-port bridge having a first port connected to the output port ofthe cable network tester, a second port connected to the input port ofthe cable network tester, and a third port connected to the cable TVnetwork, for sending the probe pulses from the output port into thecable TV network, while simultaneously sending a copy of the probepulses directly into the input port for combining therein with thereturn signal from the cable TV network.
 14. A cable network tester forlocating a fault in a cable TV network, comprising: a signal transmitterconfigured to generate a pulsed probe signal and to launch said pulsedprobe signal into the cable TV network, wherein the pulse probe signalcomprises a probe pulse that is characterized by a probe signalfrequency that sweeps across a pre-defined probe frequency band over theduration of the probe pulse; a signal receiver configured to receive areturn signal from the cable TV network and comprising a matched filterthat is matched to the probe pulse for filtering the return signaltherewith and for amplifying in magnitude one or more echoes of theprobe pulse in the return signal; and, a processor configured to analyzethe return signal to identify one or more peaks therein corresponding tothe one or more echoes of the probe pulse reflected at a fault locationin the network.
 15. The cable network tester of claim 14, wherein thesignal transmitter is configured to generate a sequence of the returnpulses, and wherein the processor is configured to compute an average ofa plurality of slices of the return signal immediately following thetime of transmission of each probe pulse from a plurality of probepulses.
 16. The cable network tester of claim 14 wherein the signaltransmitter comprises an output port, the signal receiver comprises aninput port, and the cable network tester further comprises a three-portcoupler configured to connect to the input ports and the output portsand to the cable network for launching the pulsed probe signal into thecable network and for receiving the return signal therefrom, wherein thethree-port coupler further connects the output port to the input portfor sending a copy of the pulsed probe signal from the output port ofthe signal transmitter directly into the input port of the signalreceiver for providing a timing reference for the return signal.
 17. Thecable network tester of claim 14, wherein the signal transmitter isconfigured to generate a sequence of the return pulses that issynchronized to a codeword frequency used by the cable network indownstream transmission.